It has recently become common for breweries to sell beer in bottles that are sealed with twist-off crown. This is distinguished from the pry-off crown that was utilized virtually exclusively for many years. Because the twist-off crowns are applied by machinery, it sometimes happens that the torque necessary to "break" the crown loose is greater than certain individuals can apply.
An attempt has been made in the prior art to provide a means by which the twist-off crowns can be more easily removed. Canadian Industrial Design Registration No. 33014 issued on Oct. 19, 1970, discloses a beer bottle having a central recess in the bottom or base of the bottle, the recess being provided with 7 inwardly projecting, equally spaced ribs of a pointed, triangular shape, these being adapted to enter into peripheral indentations in the crown of another bottle, whereby one bottle may be used to remove the crown from another, by the principle of mechanical advantage.
The earlier Canadian Industrial Design Registration, however, suffers from an important drawback. As illustrated in the registered design, the ribs are extremely pointed, having a true triangular cross section with an acute angle at the apex, which makes them very susceptible to stress concentration when the bottle of which they are a part is used to remove the crown from another bottle. It appears that the designer of the bottle which is the subject of the industrial design registration had in mind the idea of causing the pointed ribs to bite into the crown of another bottle, in order to obtain a secure grip on the crown and avoid slipping.
It has been discovered however, that such an acutely pointed configuration is particularly susceptible to cracking due to stress-concentration at the sharp-angled bends (both at the apex and at the shoulder regions), thereby rendering useless the crown openability feature of a returnable bottle. As is appreciated in addition such broken off pieces of glass may present a health hazard.